1. Field of the Invention
Embodiments described herein relate to methods and apparatus for a seismic cable.
2. Description of the Related Art
Seismic exploration operations generally utilize a seismic energy source to generate an acoustic signal that propagates into the earth. The acoustic signal is partially reflected by subsurface seismic reflectors in the earth, which may include interfaces between subsurface lithologic or fluid layers that may be characterized by different elastic properties. The reflected signals are detected and recorded by seismic receiver units located at or near the surface of the earth, thereby generating a seismic survey of the subsurface environment. The recorded signals, or seismic energy data, can then be processed to yield information relating to the lithologic subsurface formations, identifying such features, as, for example, lithologic subsurface formation boundaries.
Generally, the method for detection and recording of seismic signals is similar on land and in marine environments; however, marine environments present unique challenges presented by the body of water overlying the earth's surface. Seismic exploration operations in marine environments are typically conducted from the deck of one or more seismic exploration vessels, such as floating platforms or ships. The seismic exploration vessels typically provide storage and transportation for a plurality of seismic receiver units and associated operational equipment. Seismic exploration in deep water typically uses seismic sensor units deployed from the deck of the seismic exploration vessel to be placed on or near the bottom of a body of water. These seismic sensor units are part of systems typically referred to as Ocean Bottom Cabling (OBC) or Ocean Bottom Seismometer (OBS) systems, wherein data from a seismic survey may be received.
FIG. 1A is a schematic side view of a deployment operation from a vessel 5 using a cable 1 as known in the art. In the deployment operation, the cable 1 is paid out over a backdeck 10 of the vessel 5 from a spool, sheave or pulley, powered or otherwise, such as a cable handling device 15. The cable 1 includes a plurality of connectors 20 that must pass through at least a portion of the cable handler 15. Seismic sensor units 25 are coupled to the connectors 20 as the cable 1 passes over the backdeck 10 by personnel onboard the vessel. In the deployment operation, the seismic sensor units 25 are coupled to the connectors 20 by a lanyard 30, which may be a length of flexible rope, cable, or chain. The cable 1 with seismic sensor units 25 coupled thereto form a mainline cable that falls to rest on or near a bottom 40 of a body of water 35 to form at least a portion of a seismic array. The mainline cable may be many miles long and have over 200 seismic sensor units 25 attached to the cable 1 at predetermined intervals. After one or more mainline cables are positioned on the bottom 40 to define the array along the bottom 40, the seismic survey is performed.
FIG. 1B is a perspective view of a portion of a seismic cable 1 prior to coupling with the seismic sensor units 25 of FIG. 1A. Each of the connectors 20 typically include a body 45 that is larger than the diameter of the cable 1, and is configured to clamp or fasten to an outer surface of the cable 1. In some applications, the connectors 20 may be made of or include a metallic material, which may create noise during the seismic survey. In other applications, the body 45 includes ring-like or hook-like members 50 to facilitate quick connection and disconnection of the seismic sensor units. The cable 1 may also include a plurality of discrete cable coupling devices 55 configured to connect ends of cable sections to increase the overall length of the cable 1. After the seismic survey, the cable 1 and seismic sensor units are retrieved. During retrieval, the cable 1 is spooled or routed through a winch, reel or sheave, a pinch roller powered or otherwise, for example, the cable handler 15 of FIG. 1A, which pulls the cable 1 and seismic sensor units from the water. As the cable 1 passes over the deck of the vessel, the seismic sensor units 25 are detached from the cable 1 and the cable 1 and seismic sensor units are stowed.
As the seismic cable 1 shown in FIGS. 1A and 1B may be routed through a cable handler during deployment and/or retrieval, the connectors 20 and/or cable coupling devices 55 pose a risk of snagging, binding, or tangling the cable 1. In some cases, the ring-like or hook-like members 50 protrude from the periphery of the body 45, which may snag, bind, or tangle the cable. Further, the ring-like or hook-like members 55 create the risk of injury to personnel that may be in the vicinity of the cable, such as during a seismic sensor unit coupling and decoupling procedure.
Another challenge when using the cable 1 lies in the accurate placement of seismic sensor units 25 on the bottom 40 during deployment. The seismic survey typically entails advanced planning of the area of the bottom 40 to be explored, and placing the array typically requires accurate placement of the seismic sensor units 25 on the bottom 40 according to the plan. The seismic sensor unit placement may be challenged as the cable 1 is typically buoyant in seawater. The buoyancy of the cable 1 makes the sections between the connectors 20 vulnerable to currents and/or hydrodynamic forces, which may alter the fall or cause a drift of the mainline cable. The altered fall path may cause one or more seismic sensor units 25 to drift from the intended locational placement on the bottom 40.
Therefore, a need exists for an improved seismic cable.